Yggdrasil, the World Tree, has three roots, the third of which reaches down into Niflheim—a dark, bleak realm of ice and cold. Here, the root drinks from Hvergelmir, a bubbling spring that is said to feed all the rivers of Midgard. The Harbingers of the Yawning Void have gathered here, where the dark serpent-wyrm Nidhogg gnaws upon Yggdrasil’s root and feasts upon the dead who make their final end here—adulterers, murderers, and oath breakers. The cult has found a way to corrupt Hvergelmir with the darkness of the Void, hoping that corruption will spread to all the flowing waters of Midgard, hastening its destruction and poisoning the World Tree, weakening it to the point where Nidhogg can destroy its roots and bring all of creation crashing down into darkness.
Environs of Niflheim
A great plain of ice, devoid of plants and animals, the environment of Niflheim is inhospitable to life. In addition to the bone-chilling cold, dangerous foes lurk amid the twilit landscape. The worst of the dark souls confined to this place are transformed into tvierherjar (see Creature Codex) by Nidhogg. They do not linger in Niflheim but can be encountered on their way out, seeking to climb the massive, gnarled root that rises like a line of hills to the World Tree. More dangerous still are the spawn of Nidhogg (see Warlock 9: The World Tree). These huge, serpentine dragons lurk among Yggdrasil’s roots, feasting on the dead like their progenitor, though they will not turn away from the chance for live prey.
Shrouded in fog and nestled against the root of the World Tree is Hvergelmir. The waters of this hot spring are said to rise through the World Tree and feed all the rivers of Midgard. It is here that the Harbingers of the Yawning Void’s encampment can be found. Their ritual experiments have opened a portal to the Void and have managed to channel its dark energies through Hvergelmir and into the Midnight Spring on Midgard. They work to expand the power of their ritual and send this darkness seeping into the other waterways of the world.
Obstructions and Hazards
The first and constant environmental danger of Niflheim is the temperature. Visitors are constantly exposed to extreme cold while there, except for a limited area around Hvergelmir, and will suffer the effects of such conditions without proper gear or magical protection.
There is a variety of difficult terrain to be found in Niflheim, from plains of slick ice to fields where the ice is jagged and broken, forcing careful crossing. Huge cracks and chasms often mar the ice, which must be jumped or otherwise circumvented. Streams or rivers of icy water flow though some of these, flowing beneath the ice in various places. There are areas of thin ice, where the unwary can plunge through into the frigid water below. Sometimes the current is strong enough that a creature falling in can be swept away from the hole, leaving them trapped beneath the ice as they float along unless they can swim back to the hole they made, break out through another thin patch somewhere downstream, or are rescued by companions on the surface.
The Harbingers and the Gate
The Harbingers of the Yawning Void have made camp near the shores of Hvergelmir, a large, bubbling, hot spring. The shores of Hvergelmir are much warmer than the surrounding environs, and the fog created by the steaming waters hides the camp from view at a distance. Here a contingent of the cult, mostly satarre, led by Unterra ai-Niresh, perform the ritual to maintain and enlarge the portal beneath the roiling waters. The actual composition of the group encountered here is up to you, so feel free to add NPCs or monsters as you see fit. For example, there might be three or four frost giants among the cultists. The cultists have enlisted the aid of Verdsluker, an adult void dragon (see Tome of Beasts). The dragon is the focus of the ritual, acting as the key that opens the gate to the Void.
Handling the Event
This encounter can be a climactic, final battle between the heroes and the cult or a standalone encounter. Either way, there’s a lot here to throw at PCs, and they will likely not be able to handle it all at once without being crushed. In order to balance things and provide the PCs a chance to save the world, moderate how much is thrown at them at a time.
The ritual is ongoing and cannot be stopped without the portal closing, forcing the cult to have to start again from the beginning. This means they cannot simply drop everything to fight off attackers. So if the PCs take the fight to the dragon, they won’t be overwhelmed by the cultists coming at them from behind. However, the cultists can spare small teams from the ritual to defend the dragon, leaving enough of them to continue the ritual. For example, the heroes might zero in on the void dragon first, but the cultists notice them and send the aforementioned frost giants to the dragon’s defense, meaning the PCs have to take them out first.
Likewise, if the PCs take the fight to the cultists, they won’t have to fight them all at once as most will continue the ritual, leaving others to break off and try to stop the interlopers from interfering. Verdsluker has no vested interest in the cult or their goals, so the dragon may not even directly involve itself in the fight unless you decide that’s best to maintain an exciting battle. Killing their leader and several key figures of your choice should be the goal. That way, the knowledge of the ritual dies with those figures, meaning the cult can’t reopen the portal again, and the heroes emerge victorious. (Though surviving members of the cult may reappear to cause havoc again, whether for these same heroes or a new batch in another campaign.)